The signal to noise ratio (SNR) is used in MRI to describe image quality, the relative contributions to a detected signal of the true signal and random signals, or background noise.
A voxel with a larger volume contains more signal, and therefore has a higher SNR. Longer sampling time tends to reduce the noise, and therefore increases the SNR. In addition, the MRI hardware contributes to the SNR through the main magnetic field strength, the receive coil sensitivity and volume, and the receive chain noise performance characteristics. Finally, the tissue itself can contribute to the signal as determined by its relaxation and other characteristics that affect the specific pulse sequence being used.
In order to improve the SNR, typically, either the voxel size has to increase or the sampling time has to increase.
MRI systems which use lower magnetic fields (for example, 0.5 T) tend to result in a lower sample magnetization than in conventional 1.5 T and 3.0 T MRI systems. This also tends to result in a lower SNR in magnetic resonance (MR) images, if all other factors are held equal.
A commonly used pulse sequence in MRI is a spin echo sequence. It has at least two RF pulses, typically a 90° pulse (often called the excitation pulse) and a 180° refocusing pulse that generate the spin echo. Multi-spin echo sequences, or echo-train sequences, are similar. However, they apply multiple 180° refocusing pulses to produce multiple echoes following a single excitation pulse. A refocusing pulse is required for every echo produced.
In general, magnetic resonance images are produced over an imaging volume by selectively exciting and obtaining signals from slices of the imaging volume, using a combination of gradient fields and ‘spatially selective’ RF pulses. In a spin echo sequence, the repetition time, TR, is the time between successive excitation pulses for a given slice. The echo time, TE, is the time from the excitation pulse to the echo maximum. Each slice is excited and repeatedly refocused by a train of 180-degree refocusing pulses, with data sampling following each of the refocusing pulses. As such, multi spin-echo sequences may be used to efficiently acquire T2-weighted images.
Multiband encoding involves the excitation of multiple slices simultaneously within one TR time period, and the summed signal from this group of slices is typically sampled following each refocusing pulse. A pulse that is configured to perform this multiband encoding may be referred to as a multiband pulse.
Simultaneous multislice imaging using multiband RF pulses have been used in the past to excite multiple frequency bands of magnetization, with the spatial profile of multiple receiver coils used to separate the signal from each respective frequency band from the multi-band signal. However, this method is mainly used to enable more slice coverage in the same imaging time, a form of acceleration and is not used to increase SNR.